Star Wars: Rogue One - 10 Major Plot Clues From The Final Trailer
2. And He Is The Mission
The purpose of the Rebels, and what the film has been sold as showing, is to steal the plans to the Death Star in order to learn how to destroy it.
That is certainly going to be the case at some stage of the film, and the ultimate end result. What this trailer makes clear, though, is that the initial objective isn't to steal the plans: it's to bring in Galen Erso.
The message they intercept from him is how they learn of the test of the Death Star being "imminent", and that's why they recruit Jyn. Her and Cassian will then put the team together, and set out to find and rescue her father. That's something they probably won't be wholly successful in (they'll no doubt find him, but it's unlikely they'll get him out alive), and he'll tell them how/where to get the plans to the Death Star, leading to the battle on Scarif.